feat(pomodoro): zero-dependency vanilla Pomodoro timer

[TheBeege]

What and why

Implements a self-contained Pomodoro timer web app as a set of three static files
(index.html, styles.css, app.js) with zero build tooling, zero runtime
dependencies, no CDN, and no external assets. The app opens directly via file:// and
works on any static server. Closes spec specs/2026-05-30-pomodoro-app.md.

Files changed

File	Role
index.html	Document structure, controls, ARIA live regions
styles.css	Layout, interval theming via data-interval attribute, flash animation, focus styles
app.js	All behavior: state model, drift-free ticker, controls, audio, visual cue
specs/2026-05-30-pomodoro-app.md	Spec — status flipped to Delivered
plans/2026-05-30-pomodoro-app.md	Implementation plan
adr/2026-05-30-vanilla-single-page-no-build.md	ADR — Accepted
adr/2026-05-30-walltime-drift-free-countdown.md	ADR — Accepted
adr/README.md	Index updated with both new ADRs

Intentional interface / contract changes

Greenfield feature — no existing interfaces changed.

The three filenames and their relative co-location are the observable contract:
index.html → styles.css + app.js must stay siblings for file:// to work.

ADRs added

• adr/2026-05-30-vanilla-single-page-no-build.md — Accepted: three sibling static files via classic <script defer>; no ES modules (blocked under file://), no bundler, no dependencies.
• adr/2026-05-30-walltime-drift-free-countdown.md — Accepted: remaining time derived from endAtMs = Date.now() + remainingMs on each tick rather than accumulated from tick deltas, preventing cumulative drift under background-tab throttling.

Spec / plan

• Spec: specs/2026-05-30-pomodoro-app.md
• Plan: plans/2026-05-30-pomodoro-app.md

Quality gate results

• node --check app.js — SYNTAX OK
• grep -rIn -E "https?://|//cdn|fonts\\.|unpkg|jsdelivr|cdnjs" index.html styles.css app.js — NO EXTERNAL URLS FOUND
• test ! -e node_modules — PASS (no node_modules)
• test ! -e package-lock.json — PASS
• No bundler configs present (no webpack/vite/rollup/babel configs)
• No type="module" in any script tag
• No localStorage, sessionStorage, Notification API usage
• npx eslint — blocked by sandbox auto-mode policy; node --check (syntax gate) passed cleanly as the fallback per the plan

Test plan

• Open index.html via file:// — displays 25:00, "Work", count 0, Start button
• Press Start — counts down once per second; tab title updates
• Press Pause — countdown freezes; press Resume — continues from same value
• Rapid double-click Start, double-click Pause — no double-speed, no freeze corruption
• Press Reset while running — returns to 25:00, stopped, count unchanged
• Press Skip on Work — advances to Short Break, count NOT incremented
• Let a Work interval reach 00:00 (fast-mode or wait) — count +1, beep + flash fire, Short Break loaded ready (not auto-started)
• Complete 4 Work intervals — 4th completion loads Long Break (15:00)
• After Long Break completes — Work interval loads, workSinceLongBreak resets
• Reload from any state — returns to fresh 25:00/Work/0 state
• DevTools Network tab — zero requests beyond the three local files
• DevTools Application/Storage — no localStorage entries
• Keyboard-only navigation — all buttons reachable and operable; focus ring visible
• Screen reader — interval transitions announced via live region

🤖 Generated with Claude Code